Spinal fusion is indicated to provide stabilization of the spinal column for painful spinal motion and disorders such as structural deformity, traumatic instability, degenerative instability, and post-resection iatrogenic instability. Fusion, or arthrodesis, is achieved by the formation of an osseous bridge between adjacent motion segments. This can be accomplished within the disc space, anteriorly between contiguous vertebral bodies or posteriorly between consecutive transverse processes, laminae or other posterior aspects of the vertebrae.
An osseous bridge, or fusion mass, is biologically produced by the body upon skeletal injury. This normal bone healing response is used by surgeons to induce fusion across abnormal spinal segments by recreating spinal injury conditions along the fusion site and then allowing the bone to heal. A successful fusion requires the presence of osteogenic or osteopotential cells, adequate blood supply, sufficient inflammatory response, and appropriate preparation of local bone. This biological environment is typically provided in a surgical setting by decortication, or removal of the outer, cortical bone to expose the vascular, cancellous bone, and the deposition of an adequate quantity of high quality graft material. To provide the best possible patient care, the surgeon is presented with the challenge of surgically accessing the fusion site and providing graft material to the site without causing excessive morbidity and trauma to healthy tissue.
The lumbar spine is of particular interest because of the great number of cases requiring fusion and the difficulty in treating that area. Lumbar spine fusions represent over 95% of the surgical treatment for mechanical back pain and spinal instability associated with trauma or neurological pain. The lumbar region is a challenging area for fusion because of the great mobility and mechanical stresses. Unfortunately, solid union of the effected vertebrae can not always be achieved by traditional methods. Each of the three major lumbar fusion procedures, interbody fusion, posterior fusion and intertransverse process fusion have their share of complications and pseudoarthrosis.
Of the three, intertransverse arthrodesis has been preferred because it is the most reliable method for treating deformity, instability, or painful lumbar segments. This procedure provides high rotatory and translatory stability with sufficient rigidity. Furthermore, the risk of pseudoarthrosis is low probably due to the large contiguous surface area for grafting provided by the transverse processes. In spite of its popularity and relative success, therapeutic intertransverse arthrodesis still suffers from three primary weaknesses: 1) the incidence of nonunion can be as high as 35%; 2) iliac bone graft donor site morbidity occurs in up to 30% of patients; and 3) extensive dissection of paraspinal muscles at and adjacent to the levels being fused may cause fusion disease.
Either the inter-muscular or midline approaches have been most commonly used for intertransverse process arthrodesis. In the midline approach shown in FIG. 1, muscle stripping begins at the spinous process S and proceeds across the laminae M, over the capsule of the facet f joint, down the lateral wall of the superior facet process f and out to the tip of the transverse process P. The inter-muscular approach, which is depicted in FIG. 2 begins with a sub-periosteal dissection of bone at the lateral border of the superior facet process f. In both approaches, the posterior primary ramus of the segmental nerve root must be divided along with the vascular tree which accompanies it. Pressure necrosis of the paraspinal muscles can result from prolonged retraction for the deep exposure. In both approaches the incision is long and extensive muscle stripping is required at least one half level above and below the level of the intended fusion to mobilize the thick muscle mass.
These procedures, like most open spinal surgeries, require a recovery room time of several hours and several weeks of post-operative recovery time due to the use of general anesthesia and the destruction of tissue during the surgical procedure. The long incision, extensive muscle stripping, prolonged retraction of tissues, denervation and devascularization of tissue of these traditional surgical procedures cause significant trauma to the intervening tissues. Sometimes this trauma leads to persistent pain and impairment which has been labeled "fusion disease." In some cases, these invasive procedures frustrate the surgeon's purpose by causing fusion disease which can be more severe than the condition leading to the surgical intervention. Not surprisingly, many patients are reluctant to seek surgery as a solution to pain from spinal conditions because of the severe postoperative pain associated with the muscle dissection and the risks of open and prolonged spinal surgery.
Less invasive alternatives such as arthroscopic techniques have been developed to reduce pain, post-operative recovery time and the destruction of healthy tissue. Orthopedic surgical patients have particularly benefited from minimally invasive surgical techniques. The site of pathology is accessed through portals rather than through a large open incision thus preserving the integrity of the intervening tissues. These minimally invasive techniques also often require only local anesthesia. The avoidance of general anesthesia and significant tissue trauma reduces post-operative recovery time and the risk of complications. Many of these procedures are performed on the shoulder and knee on an outpatient basis with great success.
Minimally invasive surgical techniques are particularly desirable for spinal applications because of the need for access to locations deep within the body and the danger of damage to intervening vital tissues. In order to reduce the post-operative recovery time and pain associated with spinal and other procedures, various micro-surgical techniques have been developed. For example, in micro-surgical discectomies, the disc is accessed by cutting a channel from the surface of the patient's back to the disc through a small incision. An operating microscope or loupes is used to visualize the surgical field. Small diameter micro-surgical instruments are passed through the small incision and between two laminae and into the disc. The intervening tissues are disrupted less because the incision is smaller. Although these micro-surgical procedures are less invasive, they still involve some of the same complications associated with open procedures, such as injury to the nerve root and dural sac, perineural scar formation, reherniation at the surgical site and instability.
Laparoscopic and thoracoscopic techniques have been developed for anterior approaches to the thoracic and lumbosacral spine. Sofamor Danek Group markets instruments used in a laparoscopic bone dowel technique for anterior interbody fusions as an alternative to standard open anterior retroperitoneal approaches. In U.S. Pat. No. 5,195,541, Obenchain discloses a video assisted method of performing a laparoscopic discectomy using an anterior approach. These techniques typically use carbon dioxide gas as the visualization medium. Other viable alternative for lumbar procedures are needed, however, because the anterior approach is not always indicated. For example, the lumbar spine is not easily accessible anteriorly due to the danger to the great vessels and the immobility of the psoas muscle. There are also risks of complications such as bowel perforation and trauma to the ureters or the sympathetic plexus. Adhesions due to previous abdominal operations may also limit the usefulness of the anterior approach.
U.S. Pat. No. 4,545,374 to Jacobson discloses a percutaneous lumbar discectomy using a lateral approach under fluoroscopic X-ray. The advantage of this procedure is that the lateral approach does not intersect bone, nerves, blood vessels, major back support muscles or ligaments which would otherwise have to be cut or retracted. This procedure is limited because it does not provide direct visualization of the work site and both the patient and the surgeon are exposed to radiation. Although the patent discloses that the procedure is normally performed in about fifteen minutes, the total exposure time for the surgeon could be significant over a period of time. Furthermore, the patient may be exposed for a longer period of time if the surgeon is relatively inexperienced with this technique or if complications arise.
Other procedures have been developed which include arthroscopic visualization of the spine and intervening structures. U.S. Pat. Nos. 4,573,448 and 5,395,317 to Kambin disclose percutaneous decompression of herniated discs with a posterolateral approach. Fragments of the herniated disc are evacuated through a cannula positioned against the annulus. The '317 Kambin patent discloses a biportal procedure which involves percutaneously placing both a working cannula and a visualization cannula allowing simultaneous visualization and suction, irrigation and resection in disc procedures. The posterolateral approach is employed for discectomy to avoid the need for laminectomy. In U.S. Pat. Nos. 5,171,279 and 5,357,983, Matthews discloses percutaneous discectomy and subcutaneous suprafascial internal fixation methods. U.S. Pat. No. 5,439,464 to Shapiro discloses a method and instruments for performing arthroscopic spinal surgeries such as laminectomies and fusions with a mid-line or medial posterior approach using three cannulae each inserted through a separate incision.
While the development of these spinal procedures is a major step towards reducing recovery time because they require less muscle dissection, these procedure still suffer from many of the trauma-inducing disadvantages of previous spinal surgery techniques and tools. One disadvantage is that the cannulae are inserted through stab wounds in imprecise locations. Although the devices are designed to reduce tissue trauma by pushing past the muscle with a cutting or tissue moving end, the cannulae are still inserted through tissue and muscle to arrive at the working space. In most cases, nerve retraction is still required. Furthermore some of these procedures also require a gas or fluid maintained working space.
Fluid is required in some prior procedures to maintain the working space for proper function of optics fixed within a prior art cannula and inserted percutaneously. Irrigation, or the introduction of fluid into the working space, can often be logistically disadvantageous and even dangerous to the patient for several reasons. The introduction of fluid into the working space makes hemostasis more difficult and may damage surrounding tissue. The fluid environment can also make drilling difficult due to cavitation. The requirement for a fluid environment generally increases expenses associated with the surgery and adds to the complexity of the surgery, due in part to the relatively high volume of fluid required.
After the first challenge of accessing the fusion site is addressed, the second challenge is to promote bone growth at the fusion site without causing morbidity or trauma to healthy tissues. Bone graft materials are often used to promote spinal fusions. In order to effect solid bone bridging between two widely separated bones, the graft material must: 1) prevent ingrowth of fibrous tissue with a continuous layer of sufficient bulk; 2) provide a scaffold for the elaboration of bone matrix (osteo-conductive property); and 3) stimulate pleuripotential tissues to produce bone matrix and deposit thereon calcium-phosphate crystals to form bone (osteo-inductive property).
Unfortunately, the use of bone graft presents several disadvantages. Autograft, bone material surgically removed from the patient, can be undesirable because it may not be available in sufficient quantities. The structural integrity of the donor site can be compromised by the donor surgery, particularly when large amounts of graft are required. Prior arthrodesis operations can exhaust a patient's accessible supply of suitable autogenous bone. The donor surgery is traumatic to healthy tissue and increases the risk of infection and blood loss. Ironically, some patients complain that the graft harvesting surgery is more painful than the fusion surgery or the pain leading to the procedure. These complications are aggravated for lumbar fusions in which larger amounts of graft material are required.
Allograft material, which is obtained from donors of the same species, is more readily obtained. However, allografts can be disadvantageous because of disease transmission, immune reactions and religious objections. Furthermore, allogenic bone does not have the osteoinductive potential of autogenous bone and therefore may provide only temporary support, leading to pseudoarthrosis and eventual collapse of the disc space.
Bone morphogenetic proteins (BMPs) have been found to significantly reduce the time required to achieve fusion. Several recombinant human BMPs (rhBMPs) are in various stages of development and are promising bone graft substitutes. The use of recombinant BMPs will conceivably reduce recovery time by speeding the rate of bone growth and solve the problems associated with autograft and allograft, such as disease transmission, lack of osteoinductivity, immunoreactivity, as well as morbidity and trauma to a donor surgical site.
The use of any graft material requires caution. For example, when spinal fusion through the midline approach is combined with decompression or laminectomy of the spinal canal, any contaminant of one tissue space may cross-contaminate the other. Bone graft materials, in general, pose the risk of mechanical or iatrogenic complications when introduced into the epidural space. Infection in any area of the midline wound can spread to the full extent of the dissection or beyond. The safety of using BMPs in such procedures has not yet been established. It is unknown what the effect of such proteins will have in the spinal canal, intra-pleural or intra-peritoneal cavities, or retroperitoneum. Certainly there is a possibility of such complications as adhesions, arachnoiditis and retroperitoneal fibrosis.
Two major issues face surgeons in arthrodesis surgeries: exposure of the segments to be fused without causing fusion disease or other complications; and, the safety, efficacy and supply of graft materials. Therefore, a need has remained for surgical techniques which avoid the tissue trauma associated with open spinal surgery as well as risks and limitations associated with endoscopic surgery. A need has also remained for intertransverse process fusion procedures which do not cause fusion disease or other complications. A need has also remained for surgical procedures which safely use recombinant bone morphogenetic proteins in spinal fusions.